`Per Aspera Ad Astra' -- A Short Essay on the Long Quest for CP Violation

Abstract

After briefly explaining the special role played by violations of CP and T invariance and their connection with the baryon number of the Universe, I sketch the history of CP violation studies since its totally unexpected discovery in 1964. For about 30 years CP violation could be described by a single number; this has changed dramatically in the years around the turn of the millenium: (i) The existence of {\em direct} CP violation was unequivocally established in the decays of long lived kaons. (ii) For the first time CP violation was observed in a system other than that of neutral kaons, namely in $B\to \psi K_S$. The findings are in impressive agreement with the prediction of the CKM ansatz, which thus has been promoted to the status of a tested theory. These new insights were made possible by close feedback between theory and experiment as well as advances in detector design and a novel machine concept, namely that of an asymmetric collider. We also have direct {\em experimental} evidence that the observed CP violation in $K_L$ and B decays is matched by a violation of microscopic time reversal violation, as required by CPT symmetry. More recently CP violation has been observed also in $B\to \pi^+\pi^-$ and $B\to K^{\mp}\pi^{\pm}$. A few comments are added on subtle aspects of direct CP violation. While we know that the CKM dynamics are irrelevant for generating the baryon number of the Universe -- i.e. hitherto unknown forces have to be driving it -- we have also learnt that such `New Physics' is likely to contain CP violation of sufficient strength.